The present invention relates to spatially adaptive moving target indication (MTI) electro magnetic detection systems and more particularly to a radar system using autocorrelation estimation techniques to control the level of performance of the MTI circuits both spatially and with respect to time.
The following discussion assumes that the reader has a basic familiarity with the operation of radar systems, particularly MTI and Digital MTI radars. For readers desiring more background information on such subjects, reference may be had to "Radar Handbook" edited by M. I. Skolnik and published by McGraw Hill Book Company (1970).
Moving target indication (MTI) radar systems make it possible to separate moving targets from fixed targets, utilizing the Doppler effect, with techniques well known in the prior art. MTI radars typically transmit periodic pulses and make use of the variations of the phase between the transmitted wave and the received echo wave from one pulse to another pulse to detect moving targets. In such radar systems, the phase of the transmitted wave is stored or remembered during each pulse repetition period and then compared with the phase of the echo signal. The relative phase is constant for one repetition period to the next repetition period for waves reflected from fixed targets, while the phase varies with time for the case of waves reflected from a target moving with a radial velocity greater than zero with respect to the antenna. A phase detector connected to compare the remembered phase reference signal for each repetition period with the received signal produces constant amplitude pulses for fixed targets and pulses whose amplitudes vary sinusoidally at a frequency FD (generally called the Doppler Frequency) for moving targets. The Doppler Frequency depends upon the radial velocity V.sub.R of a given moving target and upon the transmitted wavelength .lambda., according to the formula FD=2V.sub.R /.lambda..
The spectrum of the constant amplitude pulses corresponding to fixed targets consists of lines spaced at frequencies 0, F, 2F, 3F . . . , nF, wherein F is the transmitted repetition frequency (PRF). The spectrum of pulses corresponding to moving targets comprises spaced lines of the type nF.+-.FD where n=0, 1, 2, 3, . . . .
In order to make a discrete velocity determination, it is necessary to know the return signal spectrum for a corresponding moving echo. The location of the echo, i.e., its bearing and range, are not derived from the spectrum. The echo bearing is taken at the antenna bearing at the time of reception of the pulses. The echo range is determined by the elapsed time between the echo transmission and the pulse reception.
In coherent pulse Doppler radar, it is desirable to provide processing of the phase discriminator circuit output signals to recover range information. For this purpose, the signal is subdivided into joined increments of range, called range bins, each corresponding to the signal returned from a discrete area located at a predetermined distance from the radar. The signal in each such bin may be analyzed for the presence or absence of a moving echo, such signal analysis consisting of the examination of the signal spectrum. Various approaches are available for this type of analysis, one of these consisting of computation of the Fourier transform of the signal and another consisting of filtering the signal with a filter having notches at 0, F, 2F, 3F, . . . such that the response from fixed targets is excluded.
Referring now to FIG. 1, there is shown a radar target 2 in a target cell. The radar set is located at reference numeral 1 and typically scans the horizon azimuthally as well as in elevation. The target cell is defined here by the range bin size, .DELTA.R, as well as by the elevation window .DELTA..theta..sub.EL, and by the azimuth window .DELTA..theta..sub.AZ. The size of the target cell depends upon several factors, including the radar type and the processing techniques used. Radars having excellent elevation and azimuthal sensitivity include the so-called "pencil beam" radars which can emit a beam having a cross section of about 1.degree. in azimuth as well as elevation. Thus the elevation and azimuth windows have the same general dimensions. The range bin is a function of the shortness of the radar burst; however, it is also well known to use pulse compression techniques with longer pulses to effectively shorten the range bin. In this respect, it has been known to use radar pulses comprising a plurality of individual segments whose phases are varied in a predetermined fashion. Using such techniques, the location of a target may be determined with high accuracy.
In addition to the return from the target 2 in FIG. 1, returns are also received from ground objects such as buildings, mountains, trees and other fixed objects. This return is known as the ground clutter return. Returns may also be received from natural phenomena such as rain and man-made chaff which may exist in the atmosphere above ground level. Additionally, electronic counter measures (ECM), such as jamming, may be aimed at the radar system and displayed or interpreted as a radar return. The radar should be capable of accurately locating and finding the target while ignoring returns from ground clutter and sky clutter. It should also preferably be capable of utilizing electronic counter-counter measures (ECCM) when ECM is encountered.
Prior art radar sets have worked relatively well at rejecting ground clutter, but have not been nearly so effective at rejecting sky clutter or using ECCM without operator intervention. The reason for the loss of effectiveness against sky clutter can be seen by referring to FIG. 2 wherein various types of existing digital MTI systems, as well as some of their advantages and disadvantages are described. The "zero reference canceller" radar, which employs the aforementioned notched filter, is effective for rejecting ground clutter because ground clutter returns have almost no phase shift change from pulse to pulse due to the generally fixed locations of the clutter and the radar transmitter. Of course, a moving radar transmitter would impose a known phase shift on the ground clutter which would be dependent upon the speed of the transmitter as well as the direction travelled with respect to the ground clutter being observed. This known phase shift could be accounted for by appropriate selection of the notched filter. However, assuming a fixed transmitter, the ground clutter still can have a phase shift associated with it due to trees and the motion of the antenna scanning past fixed objects. Thus, in FIG. 2, the ground clutter is shown over a small phase shift about zero. Typical prior art notched filter MTI systems are effective in rejecting such ground clutter.
Sky clutter, on the other hand, is a different matter because wind may have fairly high velocity, especially, for example, in the midst of a rain squall. Should a rain squall having a radial velocity of 12 meters per second, for example, occur within the target cell under examination, then a return signal spectrum, such as that shown for the sky clutter in FIG. 2, would occur. This return typically cannot be filtered out by a normal fixed MTI filter. As can also be seen in FIG. 2, "dual offset cancellers," i.e., two fixed filters, one capable of being offset, are more effective against sky clutter than the zero reference canceller; however, high performance MTI systems based on near optimal narrow band filtering techniques, implemented by such techniques as Fixed Impulse Response (FIR) filters or weighted Fast Fourier Transform (FFT) processing, yield superior results against both ground and sky clutter which are typically referred to collectively as bimodal clutter. The use of high performance MTI, such as FIR or FFT MTI, or even dual offset canceller MTI, has a serious drawback if it is used against sky clutter over the entire spatial coverage area of the 3D radar system. This drawback will subsequently be discussed with reference to FIG. 6.
Referring now to FIG. 3, there is shown pictorially the coverage diagram of a typical radar system. MTI processing techniques are generally employed only for lower elevation angles and in the shorter slant range distances. MTI is not needed at elevations greater than four or five degrees because of the lack of ground clutter at those elevations nor is it needed for distances much greater than 100 kilometers because of the curvature of the earth taking ground clutter out of the slant range of the radar system. Furthermore, MTI is preferably only used when necessary because MTI filters out targets moving circumferentially with respect to the transmitter and targets having a radial velocity which produces a spectral line at either 0, F, 2F, 3F . . . , nF. These speeds are referred to as blind speeds for MTI systems (see Skolnik, Section 17.2).
In FIG. 3, it should be noted that the desired detection range of the radar system is typically greater at low elevations than at high elevations due to the effect of the earth's curvature and the fact that targets are not expected to be found at heights greater than about 25 kilometers. For this reason, the Pulse Repetition Frequency (PRF) may be higher at the higher elevations. To obtain more energy on a target at the lower elevations, a longer phase modulated burst may be used compared to the phase modulated bursts used at higher elevations. The effect of these considerations may be seen in FIG. 4 where the scan program for a three dimensional radar system is shown. The short vertical lines which subtend approximately five degrees indicate five essentially simultaneous beams, the beams having slightly different frequencies associated therewith such that when a pencil beam radar wave is emitted from a frequency sensitive antenna, five or so separate pencil beams may be emitted, thereby obtaining approximately five degrees elevation coverage from this multiple beam pulse. After each pulse there occurs an interval of time on the order of two to five milliseconds during which time the radar listens for return echos. The listening time is greater at lower elevations than higher elevations due to the greater slant range desired of the radar set at lower elevations than at higher elevations, as aforementioned. Also, due to the fact that MTI processing is usually desired at lower elevations and assuming a zero reference canceller is used to do this MTI, three consecutive pulses are used for analysis in the notched filter (see FIG. 2). At the higher elevations, that is, above five degrees, no such MTI processing is done and therefore single pulses are shown in these elevations.
Referring now to FIG. 5, there is shown the front end equipment of a conventional pencil beam radar system. The radar system shown in FIG. 5 employs five beams which are emitted from frequency-sensitive antenna 30. These five pencil beams are emitted one right after another, the five beams comprising a radar pulse. The RF energy in each beam is preferably phase modulated to improve range sensitivity of the radar system as aforementioned and a slightly different carrier frequency is associated therewith, thereby permitting antenna 30 to alter the elevation of the beam in response to the frequency shift occurring between the packets of RF energy in a radar pulse. Thus, the RF energy applied to transmitter 32 is shown at numeral 31 comprising packets F1-F5, each of which has a slightly different carrier frequency and each of which is preferably phase modulated. Echo returns are received by antenna 30 during the interpulse period and applied via duplexer 33 to receiver 34 where the return information is separated into the five carrier frequencies to obtain five different responses for a given radar pulse.
This radar system of FIGS. 1-5 works well against ground clutter but is not capable of rejecting sky clutter for the reasons aforementioned. Of course, a narrow band filter bank is capable of rejecting sky clutter, however, as can be seen from FIG. 6, a scan program which routinely used this technique at all elevations could not scan the desired volume within a relatively small diameter beam because of the number of consecutive pulses (for example, eight as shown in FIG. 6) required for the filter to perform the analysis. Thus, in FIG. 6, a pure narrow band filter scan program only can complete the lower elevation analysis and is blind to targets having an elevation of 15 degrees or greater. Alternatively, a pure narrow band filter scan program utilizing a fast PRF to obtain the desired elevation scan would lose range capabilities.
It is, therefore, one object of this invention to provide a radar system capable of using high order MTI techniques, such as narrow band filtering, only where needed to reduce clutter.
It is another object of this invention to provide a radar system with a mechanism for employing narrow band filtering to accomplish near optimal filtering against the encountered environment using a measured correlation coefficient as a control mechanism for selecting the narrow band filter bank to be used.
It is yet another object of this invention that the measured correlation coefficient be used to determine the order of the filter bank (i.e., the number of filters used).
It is another object of this invention to develop a go/no-go test to be used for invoking ECCM techniques against not highly correlated clutter-like returns not removed by the normal fixed filter MTI.
It is yet another object of this invention that the use of high order MTI filtering techniques should not restrict the elevation or range coverage of the radar system.